Field of the Invention
The present invention relates generally to apparatus, systems, and methods for noninvasively measuring cardiac output (CO) and left ventricular stroke volume (SV). More specifically, the invention includes systems and methods that measure CO and/or SV using plethysmographic waveform data collected by a pulse oximeter.
Description of Related Art
There has been a long felt need in the medical arts for a noninvasive way to measure left ventricular stroke volume (SV) and cardiac output (CO). SV is the volume of blood pumped by the left ventricle of the heart with a single heart cycle. CO is the product of SV and hear rate (HR). SV and CO are important physiological parameters for a number of medical conditions, including congestive heart failure (CHF).
Patients being treated for heart failure are normally medicated with drugs that regulate diuresis and heart muscle function. An important aim of drug therapy is to maintain a CO that is sufficient to perfuse tissues with oxygenated blood. It is advantageous to use the lowest possible doses of drugs to manage CHF because the drugs used produce unwanted side effects. To optimally manage the dosage and selection of drugs in the treatment of CHF, one must monitor CO to assess the efficacy of the drugs and dosages being administered and/or to monitor patient compliance.
Currently, CO is measured using invasive techniques such as the Fick method, the Thermodilution method, and implantable microelectromechanical devices (MEMs), also called CardioMEMs. The Fick method involves the measurement of oxygen consumption and computing the arteriovenous difference using samples of arterial blood and mixed venous blood from the pulmonary artery. The thermodilution method measures the rate at which cold saline solution is diluted in the blood. Both of these methods are performed in a hospital setting because they require the placement of a catheter in the pulmonary artery. Additionally, the use of a pulmonary artery catheter use may also increase morbidity in critically ill patients. CardioMEMs are surgically implanted into patients in a hospital setting and, once implanted, provide measurements of SV and CO that can be used to monitor patients. The implantation of the cardioMEMs device into the pulmonary artery, however, is expensive and is performed in a hospital setting and involves the risks associated with heart catheterization.
Less invasive techniques for measuring SV and CO include esophageal Doppler and transesophageal echocardiography. Esophageal Doppler measures blood flow velocity in the descending thoracic aorta using a flexible ultrasound probe that is inserted into the esophagus. The blood flow velocity is combined with an estimate of the cross-sectional area of the aorta estimated from the patient's age, height, and weight to calculate SV and CO. This technique requires someone with technical skill to insert an esophageal Doppler monitor, which must be properly aligned with respect to the thoracic aorta to provide accurate measurements. Transesophageal echocardiography involves measuring SV using flow velocity calculated from the area under the measured Doppler velocity waveform at the pulmonary artery, the mitral valve, or the aortic valve. This technique requires a highly trained operator to position and place the esophageal Doppler monitor. These procedures are not truly noninvasive because accessing the esophagus is perceived by patients as invasively uncomfortable.
Truly noninvasive methods, apparatus, and systems are needed that can measure SV and CO, preferably in the homes of patients without the need for skilled caregivers. Pulse oximetry has been investigated for decades as a possible tool for the noninvasive measurement of SV and CO. A pulse oximeter (PO) is a device that obtains photoplethysmography (PPG) data, which measures changes in blood volume within a tissue caused by the pulse of blood pressure through the vasculature in the tissue. The blood volume change is detected by measuring the amount of light transmitted or reflected to a sensor from a light source used to illuminate the skin. The shape of the PPG waveform varies with the location and manner in which the pulse oximeter is contacted with the body. In addition to PPG data, a pulse oximeter measures peripheral oxygen saturation (SpO2). Most often, the device operates in a transmission mode in which two wavelengths of light are passed through a body part to a photodetector. Changes in absorbance at each of the wavelengths are measured, which allow the determination of the absorbance due to pulsing arterial blood, excluding venous blood, skin, bone, muscle, and other tissues. Alternatively, reflectance pulse oximetry can be used. A typical pulse oximeter comprises a data processor and a pair of light-emitting diodes (LEDs) facing a photodiode. One LED produces red light having a wavelength of 660 nm, the other infrared light having a wavelength of 940 nm. Oxygenated hemoglobin absorbs more infrared light and less red light than hemoglobin. The transmission signals fluctuate over time because of changes in the amount of arterial blood present in the tissue caused by the blood pulse associated with each cycle of the heart. The ratio of red light measured to infrared light measured is calculated by the processor and is converted by the processor to SpO2 using a lookup table based on the Beer-Lambert law.
Awad et al. (J. Clinical Monitoring and Computing, 2006, 20:175-184) reports that researchers have been attempting to understand the relationship between central cardiac hemodynamics and the resulting measured peripheral waveforms. Awad et al. studied ear pulse oximeter waveforms in order to understand the underlying physiology reflected in these waveforms and to extract information about cardiac performance. Multi-linear regression analysis of ear plethysmographic waveform components were used to estimate CO from the ear plethysmograph and it was determined that ear plethysmographic width correlates with CO. Awad et al. does not suggest that this correlation allows pulse oximetry or plethysmographic data to be used to calculate SV or CO.
Natalini et al. (Anesth. Analg. 2006, 103:1478-1484) reports the use of pulse oximetry to predict which hypotensive patients are likely to respond positively to increasing blood volume. Arterial blood pressure changes during mechanical ventilation are reported as accurately predicting fluid responsiveness. Photoplethysmographic (PPG) waveform variations measured by pulse oximetry showed a correlation with measured pulse pressure variation values associated with fluid responsiveness but neither SV nor CO were calculated. Natilini does not indicate that SV or CO can be calculated using PPG data.
US 2013/0310669 discloses a method for determining mixed venous oxygen saturation (SvO2) using a photoplethysmography pulse oximeter (PPG PO) device that measures changes in pulmonary circulation using a light source and a light detector applied to the thoracic wall of a patient. The light source and the detector are separated by at least 15-20 mm so that the region of illumination overlaps a portion of the pulmonary microcirculation beneath the PPG device. The contribution of circulation in the thoracic wall to the PPG signal must be assessed using an additional detector and/or light source that is attached to the thoracic wall less than 8 mm apart. The SvO2 value can be used to calculate CO using the Fick method from the values of total oxygen consumption, arterial oxygen content and venous oxygen content. One drawback of this method is that the pulmonary microcirculation is surrounded by bone, muscle, and other tissues that make the contribution of circulation in the thoracic wall to the PPG signal difficult to measure reliably. Another drawback is that the measured values depend on the accurate placement of the light emitters and sensors, which may be difficult to reproduce for each subsequent measurement.
U.S. Pat. No. 9,289,133 discloses a method and apparatus for monitoring proportional changes in CO from a blood pressure signal measurement obtained by fingertip PPG. A time constant of the arterial tree is defined as the product of the total peripheral resistance (TPR) and a constant arterial compliance and is determined by analyzing long time scale variations of more than one cardiac cycle. A value proportional to CO is determined from the ratio of the blood pressure signal to the estimated time constant using Ohm's law. An invasive, absolute CO calibrating measurement is required to derive absolute CO values from the proportional CO change values obtained using PPG. This method and apparatus cannot measure values for SV or CO without an invasive CO measurement and therefore requires a hospital setting and skilled medical personnel.
WO 2010/0274102 A1 discloses a data processing method and associated apparatus and systems that measures SV and CO from pulse oximetry data. The pulse oximeter system comprises a data processor configured to perform a method that combines a probabilistic processor and a physiological model of the cardiovascular system in a Dynamic State-Space Model (DSSM) that can remove contaminating noise and artifacts from the pulse oximeter sensor output and measure blood oxygen saturation, HR, SV, aortic pressure and systemic pressures. This pulse oximeter and associated method provides truly noninvasive measurement of CO and SV suitable for monitoring patients with CHF. The DSSM comprises a mathematical model of the cardiovascular system that models the physiological processes which produce the pulses measured by the pulse oximeter. In one embodiment, the model comprises parameters including aortic pressure, radial pressure, peripheral resistance, aortic impedance, and blood density.